BD9470AFM・BD9470AEFV

SS

REG58

Css[F]

Iss[A]

4.7V＜Vss

⇒FB Output Current

=Source Only

V

V

3. Application of BD9470AEFV・BD9470AFM

3.1 BD9470AEFV・BD9470AFM examination for application

●Start-up and SS capacity setting explanation

This section described the start-up sequence of this IC.

○Description of start-up sequence

①STB=PWM=ON

②System is ON.SS starts to charge.

At this time, a circuit in which SS voltage for slow start is equal to FB voltage regardless of whether the PWM pin is set to

Low or High level.

③Since the FB pin and SS pin reach the lower limit of the internal sawtooth wave, the DC/DC converter operates and

VOUT voltage rising.

Until it reachs a certain voltage even PWM=Low by vlotage maintenance function.

（For detailed OVP maintanence function, please refer to”VOUT(OVP) maintanence function section”.）

⑤Vout voltage continues rising to reach a voltage at which LED current starts flowing.

⑥When the LED current reaches the set amount of current, isolate the FB circuit from the SS circuit. With this, the start-up

operation is completed.（Fast start-up is also diasabled by VOUT maintanence function）

⑦After that, conduct normal operation following the feedback operation sequence with the LED pins.

If the SS pin voltage reaches 3.7V or more, the LED protection function will be activated to forcedly end the SS and

FBequalizing circuit.

○ SS capacity setting method

Boot system as above described, because of start-up in the state of FB=SS, the start-up time can be imaged of the time to

reach the point from the feedback voltage FB from STB = ON.If you SS> 4.9V, FB output current mode will become Source

mode operation.

If the feedback voltage of FB is the same as VSS and the time can be calculated as below.

]

[

]

[

2

]

[

]

[

Sec

A

V

VFB

F

C

T

ss

ss

µ

×

=

Figure 15. Timing chart of start-up

Figure 16. SS setting procedure in FB Source mode

STB

SS

SLOPE

PWM

N

VOUT

ILED

LED_OK

①

⑦

⑥

③

②

⑤

FB

OSC

④

LED_OK

SS=FB

Circuit

SS

FB

5V

0.3～

0.519V

LED_DRIVER

OSC

DRIVER

COMP

N

LED

VOUT

PWM

SS

SLOPE

D

Q

PWM

Css

OVP

KEEP

OVP

LED_OK

Time

SS=FB

Finished Start Up

VSS>4.9V FB=Source Mode

4.9V

13/35

TSZ02201-0F10C1002000-1-2

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19.Oct.2013 Rev.003

www.rohm.com

TSZ22111・15・001